1. Field of the Invention
The present invention relates to a lens for reading an original, a method and an apparatus for reading an original, and an image forming apparatus.
2. Description of the Prior Art
Conventionally, an apparatus for reading an original has been widely utilized as an original reading unit of a facsimile machine and a digital copying machine, and an image scanner. The apparatus reduces an original image to be read by a lens for reading an original, and converts the original image to a signal by imaging it on a line sensor of a CCD or the like.
When a color original image is read out with a full-color, a 3-line CCD in which a light receiving element having a filter for decomposing colors for red, green, and blue is disposed in three lines on one chip is used, and the color original image is imaged on a light receiving surface of the light receiving element, then the color original image is discomposed into the three primary colors by the filter to convert it to a signal.
Comparing to a photographing lens, for the lens for reading an original used for the device for reading an original, generally, a high contrast in a high spatial frequency area and a substantially 100% of an aperture efficiency until a peripheral portion of a filed angle are required. Moreover, a bright lens in order to achieve a high-speed reading is also demanded.
In order to read the original image accurately, for the lens for reading an original, a number of aberrations should be successfully corrected.
Especially, a distortion is relatively allowed for the photographing lens; however, the distortion should be sufficiently corrected for the lens for reading an original.
In order to read successfully color image information with the full-color, a chromatic aberration for each color as well as a currature of filed should be highly sufficiently corrected because an image of the original image by the lens for reading an original is required to be imaged by coinciding an image-forming position of red, green, and blue toward an optical axis on the light receiving surface of the 3-line CCD.
As the lens for reading an original, a Gauss type of four groups for six lenses has been widely known in the prior art (for example, Japanese Patent Laid-Open Hei6-109971, Hei10-68881, Hei10-253881, and Hei11-109221)
Generally, the Gauss type lens is capable of successfully correcting the currature of field up to about 20 degree of a half filed angle, and reducing a generation of a coma flare even though an aperture is relatively large. As a result, the high contrast in the high spatial frequency area until the peripheral portion of the image surface is able to be received. However, the Gauss type is composed of 6 lenses, which is a lot of numbers of lenses, and contributes to a large external diameter of lens. As a result, it becomes difficult for the lens for reading an original to miniaturize, and also there is a limit for miniaturizing and lowering a cost for the device for reading an original and the image forming apparatus.